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Lead-free: PCB Test and Inspection
December 31, 1969 |Estimated reading time: 8 minutes
Much has been written regarding the Effects of lead-free solder on the assembly process. However, it is still unclear how alternative solder materials will affect test and inspection methods. This article examines the issues and trends associated with lead-free solder quality and PCB inspection techniques.
By Paul R. Groome
The mandate to manufacture lead-free PCBs is quickly approaching. Much has been written about the effect of lead-free solder on the assembly process, although how alternative solder materials will affect test and inspection methodology is not well documented. Lead-free solder introduces new, problematic test and inspection issues.
Lead-free solder has a rougher surface finish and generates a different-shaped fillet. It also is more prone to voids and tombstoning. These and other deviations can require adjustments to commonly used inspection techniques, such as automated optical inspection (AOI). While the results of a National Physical Laboratory (NPL) study confirm that AOI systems can be used to inspect lead-free surface mount assemblies, many defects created by lead-free processes are not visible. The added loss of visual and electrical access due to the growing complexity of PCBAs compounds the problem.
Lead-free board assemblies also challenge laminography-based automated X-ray inspection (AXI) equipment. These systems rely on mechanical apparatus and use image-averaging techniques that reduce image quality and the effectiveness of finding faults. The result is less distinction between good and bad solder joints, balancing the consequence on reliable defect detection and the risk of shipping bad product.
This article examines the issues and trends of lead-free solder quality and PCB inspection methods. It introduces a computationally based 3-D AXI technique designed to achieve false, failure-free inspection to detect subtle voids and other lead-free solder defects accurately, even as board complexities grow.
Lead-free Solder Effects
For all solders, the critical point in manufacturing is the reflow process. The typical 60/40 eutectic tin/lead solder mixture has a melting point of about 183°C, so reflow temperatures of 220° to 230°C are common. The iNEMI-recommended lead-free solder is 95.5% tin, 3.9% silver and 0.6% copper with a melting point of 217°C. This means that peak-reflow temperatures as high as 260°C probably will be required. Temperatures can be reduced by a longer pre-heat, but this risks increased void formation. Materials and components must be qualified for higher temperatures. High-temperature warpage of board materials and component packages, especially BGAs, will be a particular concern.
Figure 1. Lead-free solder is more prone to tombstone failures due to higher coalescent (surface-tension) forces.
Lead-free solder presents increased risk of several types of failures. It is more prone to tombstone failures, a phenomenon in which a difference in surface tension at one end of a component during melting/solidification will cause the part to stand up on one end (Figure 1). Because lead-free solders have different wetting profiles, components are not as likely to self-correct during reflow in the event of misalignment during placement.
Lead-free and AOI
Lead-free solder also has several characteristics that, while they generally do not cause defects, can complicate optical inspection (Figure 2). Lead-free solder joints typically are more striated and rougher than leaded joints due to the phased transition from liquid to solder. Lead-free solder also has a higher surface tension and does not flow as readily as leaded solder, causing a slightly different-shaped fillet.
Figure 2. Due to the phased transition from liquid to solid, lead-free joints tend to be more striated and rough than corresponding leaded joints.
A 2002 NPL study independently evaluated the ability of AOI to inspect lead-free solder joints. The study evaluated 15 conventional leaded and lead-free target boards, with and without defects, using identical algorithms. Study results demonstrated that most AOI systems could be used to inspect lead-free surface mount facilities. It is also clear that AOI systems will require considerable adjustment to address varying defects and surface conditions found in lead-free manufacturing.
Lead-free Solder and ICT
Lead-free solder also presents challenges to in-circuit testing (ICT). Concern exists that increased flux residues from lead-free solder will build on the probe tip, increasing contact resistance. This likely will require new probe styles, more aggressive cleaning schedules and shorter probe-replacement cycles. It also is important to note that the increased brittleness of lead-free solder may cause it to be damaged by excessive board flexing on the fixture. This would require new probe shapes and styles. iNEMI and other organizations are evaluating the effects of the tendency of lead-free tin alloys to grow whiskers between joints due to increased reflow and solder being placed under compression ICT.
AOI and ICT systems cannot detect voids. This is because flux trapping and longer reflow profiles have a tendency to cause smaller voids to coalesce. The affect of voids on the reliability of solder connections is a subject of some controversy. The IPC-A-610 C standard states that the maximum acceptable percentage of the ball-to-board interface area covered by voids should not exceed 10%; and joints with more than 25% voiding are classified as defects. The committee developing the IPC-732 standard is tasked with the goal of resolving what level of voiding is acceptable.
Other factors also affect PCB inspection. Increases in component density are making it difficult to use ICT. Average density of wireless devices has reached a point where ICT cannot be used effectively. Higher speeds also reduce electrical access because electrical test points affect signal integrity above 500 MHz. The move to area-array packaging is driving visual loss of access. The proportion of joints that cannot be inspected with AOI solutions is expected to rise to about 50% by 2007.
Strengths and Weaknesses of Conventional AXI
For these reasons, more attention is focused on AXI methods. The use of lead-free solder has little effect on AXI systems (Figure 3). While lead-free solder is about 15 to 20% less dense than leaded solder, AXI has no problem imaging lead-free materials. AXI can see the full shape of the joint, make volume measurements, provide full heel-and-toe measurements and measure angles based on slope of solder density. In particular, AXI has the ability to easily detect voids, a capability that may become more important in the future. The loss of electrical and visual access, a side effect of recent manufacturing trends, does not affect AXI.
Figure 3. AXI has no difficulties imaging lead-free materials.
Recent manufacturing trends that have highlighted the benefits of AXI also have highlighted some weaknesses. The traditional approach to AXI is based on laminography, which requires relative motion of the X-ray source, detector and board. The use of a rotating source and mechanical rotating detector requires accurate board movement in the Z-axis. Laser mapping is required to determine board height. Only points from the focal slice are projected at the same location on the detector and sharply imaged. Object structures above and below the focal slice are not sharply imaged, leading to low-contrast resolution. The combination of mechanical movement and low resolution raises false-call rates to typically between 2,000 and 10,000 parts per million joints (ppmJ). The need for mechanical movement and laser mapping adds to inspection cycle times, making it impossible for laminography systems to operate at the beat rate.
Off-center Tomosynthesis
A newer approach - off-center tomosynthesis - can overcome these problems by using a steerable X-ray source and a large, flat-panel detector. By sequentially moving the X-ray beam position and the viewpoint of the detector, a series of images are acquired at different angles. Generally, at least one of these images will provide an unobstructed view of every solder joint (Figure 4). For example, a slice can be obtained that removes all bottom-side components, so solder connections on the topside can be inspected easily. Because the multiple images required to obtain an unobstructed view of every solder connection are captured simultaneously rather than sequentially, this approach operates at a faster speed (Figure 5).
Figure 4. Off-center tomosynthesis (left) provides high resolution without loss of original image information relative to 2-D AXI (right).
The nine images captured in a commercial implementation come from nine separate fields of view that are later reconstructed to generate 3-D slices in a manner similar to that of computer tomography used in medical imaging. The use of static-image capture in off-center tomosynthesis increases image quality. Eliminating moving parts and laser mapping reduces cycle time, making it possible to operate at beat rates without difficulty. The use of computational techniques to reconstruct images eliminates artifacts. Removing mechanical errors and averaging reduces false calls to below 500 ppmJ.
Figure 5. Off-center tomosynthesis image reconstruction.
Keeping false calls below 500 ppmJ can reduce the number of operators required for inspection, reduce required operator skill levels and training requirements and improve operator-confidence levels. A study of 100 operators using an inspection system with 4,000 ppmJ false-call rates showed that only 3 in 100 operators were able to detect all defects; and 57% of all defects reported were allowed to escape. Using a $40/hr. fully loaded operator cost, an off-center tomosynthesis AXI system with a 500 ppmJ false-call rate will save $1/board in high-volume automotive products, $2/board on medium-volume server boards and $7.50/board in low- to mid-volume data communications products.
A Blended Strategy Maintains Yields
As products become more complex, the number of solder joints per board increases. If defect rates per solder joint remain constant, yield will be reduced. At a defect rate of 100 ppmJ, an increase in the number of solder joints from 5,000 to 10,000 per board will reduce yield from 65 to 38%. Total defect capabilities (TDC) for each inspection/test technology can be calculated for each defect type. These calculations show that ICT alone provides 53% TDC, AOI post-place plus full ICT provides 61% TDC, AOI post-reflow plus full ICT provides 81% TDC and AXI plus full ICT provides 94% TDC.
Lead-free manufacturing adds a new dimension to a challenging test and inspection environment. This provides new challenges for AOI and ICT systems that were already facing difficulties. Conventional AXI systems address falling TDC rates of AOI and ICT systems; however, their throughput and false-call rates leave much to be desired. Off-center tomosynthesis may address these concerns by raising defect coverage, reducing false calls and increasing throughput to beat-rate levels.
Paul R. Groome, director of automated X-ray inspection products, Teradyne Inc., may be contacted at (858) 391-3810; e-mail: paul.groome@Teradyne.com.